Liquid jet recorder

ABSTRACT

The present device is a liquid jet recorder having a capillary nozzle out of which an electrically conductive ink jet is expelled by pressure, and a control electrode or tube coaxially surrounding the path of the jet, the wall of the tube being porous to the ink and formed of an electrically conductive material. Charging means are included for charging the jet at one potential and the electrode at another potential. The recording surface upon which the ink jet is intended to selectively write is provided with means for selectively charging the surface at a potential which repels any drops of the jet from striking the surface.

United States Patent Hertz [1s] 3,673,601 451 June 27, 1972 [54] LIQUID JET RECORDER OTHER PUBLICATIONS [72] Inventor: Carl Hellmuth Hertz, Skolbanksvagen 8,, Hertz et al.; A method for the Intensity Modulation of a 223 67, Lund, Sweden Recording lnkjet and its Applications; ACT A Universitatis [22] Filed Much 31 1970 Lundensia H 1967, No. 15.

[21] Appl. No.: 24,140 Primary Examiner-Joseph W. Hartary AnomeySchiller & Pandiscio 30 Foreign Application Priority 0m [57] ABSTRACT Apnl 2, 1969 Sweden ..4689/69 The prcscm device is a liquid jet recorder having a capillary nozzle out of which an electrically conductive ink jet is ex- U.S. Cl ..346/75 pelled by pressure and a comm] electrode or tube coaxiauy surrounding the path of the jet, the wall of the tube being [58] Field of Search ..346/75 porous to the ink and famed of an elecmcauy conductive material. Charging means are included for charging the jet at [56] Rdmnces cited one potential and the electrode at another potential. The

UNITED STATES PATENTS. recording surface upon which the ink jet is intended to selectively write is provided with means for selectively charging the l,94l,00l 12/1933 Hansell ..346/75 X rfa e at a ot ntial which repels any drops of the jet from 3,369,252 1968 strikingthe surface. 3,416,153 l2/l968 3,500,436 3/1970 5 Claim, 1 Drawing Figure 40 36 j Lu U SUCTION PUMP LIQUID JET RECORDER This invention relates to graphic recording systems, and more particularly, to a liquid jet recorder.

In my copending patent application Ser. No. 861,743, a simple electrode device is described, which allows intensity modulation of a tracing liquid jet by an electric voltage which is applied between the control electrode and the tracing liquid. If this voltage is sufficiently high, for instance more than 100 volts, the normally well-defined liquid jet is con verted into a spray consisting of small electrically charged drops. These then strike the inner porous wall of a tubular control electrode from whence they are sucked by a suction pump. In this manner, the recording line which the liquid jet makes on a recording paper, can be intensity modulated through the voltage applied to the control electrode.

The above-noted patent application fully describes the advantages which this very simple form of control electrode affords over previous embodiments shown in U.S. Pat. No. 3,416,153. However, there is also an inconvenience inapplying the teachings of the aforesaid application. If the control voltage on the control electrode is not more than about 1,500 volts, the jet is not dispersed to the extent that no drops whatsoever land on the recording paper and, thus, there is a tendency to create a general background or gray area. This can partly be prevented by reducing the opening of the control electrode facing the recording paper or by making the aperture in the control electrode cone-shaped. In spite of this, a certain background cannot be entirely avoided with moderate voltages on the control electrode.

A principal object of the present invention is to provide means for reducing the background provided by such control electrodes. Another object of the present invention is to increase the upper limit frequency of writing devices employing such control electrodes.

Other objects of the present invention will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the drawing wherein there is shown a typical device embodying the principles of the present invention.

The present invention incorporates by reference the system shown in the aforesaid patent application and therefore includes jet nozzle 20 through which fluid ink 22 is expressed along axis A-A as a jet to ultimately under certain conditions impinge upon a recording surface such as paper 24 mounted on.a movable member such as drum 25. The jet is controlled by control electrode 26 having a hollow elongated interior formed of electrically conductive material and being axially aligned with axis A-A. The interior of the electrode 26 is lined with a porous material on liner 27 backed by suction chamber 28. A suction pump 29 is connected through conduit 30 to chamber 28 for applying a net negative air pressure to the latter. The electrode is charged with a control voltage provided by a connection between liner 27 and source 34 while the fluid in nozzle 20 is kept at another voltage as by grounded contact 32.

As noted in respect to the aforesaid application, if the voltage applied across the nozzle and electrode is not more than approximately 1,500 volts, the dispersion of the jet is not sufficient to insure that all the ink impinges on the liner 27 and, thus, there is a tendency to create a background on paper 24.

However, this background can be suppressed by applying a high electrostatic charge on the surface of recording paper 24 typically of the same sign or polarity as the electric charge of the drops in the dispersed jet. This results in the drops in the dispersed jet being repelled back toward control electrode 26 and thus prevent the formation of background on the recording paper.

The above-mentioned charge can be obtained in various ways. As normal paper generally is not quite insulating, it is 1 necessary to insulate drum 25 from the environment. This is best done by making the whole drum or part thereof from an insulating material. The above-mentioned charge can thenbe applied to the paper surface by arranging one or more conductive points 36 or, in the immediate vicinity of the drum, a thin wire which is parallel to the-drum. Point 36, or the wire, is connected to source 40 of high voltage so that a corona discharge arises at point 36 orthe wire. This gives the paper surface the desired electric charge which prevents the dispersed drops from reaching the paper surface. The distance from the corona point to the recording paper is preferably about 5-10 mm. and the: voltage applied at the corona points about 8-10 kv. I

If the outer drum surface is made from conductive material, an electric voltage can be applied to this coating by means of a sliding contact. Due to the conductivity of the recording paper, paper surface 24 then will have the same potential. This system has the advantage that the voltage required to repel the dispersed drops is somewhat smaller. Of course, the sliding contact may also be applied directly so that it slides on. the paper surface proper, as shown in the drawing by the sliding contact 38 represented by dotted lines.

Further, to achieve a perfectly satisfactory result, it is important that the liquid jet is not laterally dispersed and thus increase its diameter (i.e., become a conical spray) on its way towards the recording paper when control electrode 26 is held atground potential, i.e., the same potential as the. tracing liquid. This requires that nozzle 20 be very carefully designed.

The dispersion effect can be made more efficient if the drop formation process in front of nozzle 20 can be controlled. Further, the upper'limit frequency of the writing device increases the more drops and the more uniform drops there are formed per second at the drop formation point. This can be done mechanically by vibrating the nozzle at a suitable frequency which is dependent inter alia, on the diameter of the nozzle. The simplest way of doing this is to apply a vibrator 42, e.g., a piezo-electric crystal to the vicinity of the nozzle 20, for instance, at its feed tube or mounting means, as shown in the drawing. By means of an electric oscillator circuit, the crystal is thus caused to oscillate in a known manner, while the mechanical oscillations propagate to the liquid jet at the nozzle opening and determine the drop formation.

The use of this vibrating system which preferably oscillates along the jet axis A-A, gives better resolution than that obtained by the device described in the aforesaid patent application; i.e., it allows the jet to trace or print with more accuracy in that it provides droplets with uniform fluid dynamics and thus uniform mechanical flight characteristics. In the prior art, using nozzle vibrators, the nozzle vibration frequency should be carefully synchronized to the frequencies applied to the droplet charging electrode. Of course, in the present invention, this synchronization aspect is quite unnecessary and, in-

deed, the frequency of nozzle vibration can be quite different than the frequency applied at electrode 26.

Another advantage of the present invention over the electrode device described in the aforesaid patent application is that a large number of such electrodes may be placed side by side without the electric fields from adjacent control electrodes interfering with the field. at the drop formation point of their respective liquid jets. The advantage of this in connection with writing out numbers has already been pointed out in the aforesaid patent application. It is clear, however, that the number of such writing units that can be placed beside or obliquely after each other is unlimited so that a whole surface can at the same time be provided with writing by means of a large number of liquid jets, the writing traces of which lie very closely beside each other.

What is claimed is:

1. In a liquid jet recorder for pressure ejecting along a predetermined path to a recording surface, a fine jet of-an electrically conductive tracing fluid from a capillary nozzle, and including a control electrode positioned adjacent said nozzle at the point of droplet formation, and means for selectively applying between said nozzle and electrode a potential sufficient to cause said jet to disperse into a conical spray, the

4. In a liquid jet recorder as defined in claim 1, wherein said means for applying said charge comprises at least one metallic point positioned adjacent to said surface and connected to a voltage source so as to provide a corona discharge to said surface for charging the latter.

5. In a liquid jet recorder as defined in claim 1 wherein said capillary is mechanically vibrated at a high frequency by vibrator means coupled to said nozzle. 

1. In a liquid jet recorder for pressure ejecting along a predetermined path to a recording surface, a fine jet of an electrically conductive tracing fluid from a capillary nozzle, and including a control electrode positioned adjacent said nozzle at the point of droplet formation, and means for selectively applying between said nozzle and electrode a potential sufficient to cause said jet to disperse into a conical spray, the improvement comprising means for applying to said surface an electrical charge such that said fluid is repelled from said surface.
 2. In a liquid jet recorder as defined in claim 1 wherein said electrical charge is at the same polarity as said potential.
 3. In a liquid jet recorder as defined in claim 1 wherein said means for applying said charge comprises a sliding contact positioned to contact said surface and connected to a voltage source.
 4. In a liquid jet recorder as defined in claim 1, wherein said means for applying said charge comprises at least one metallic point positioned adjacent to said surface and connected to a voltage source so as to provide a corona discharge to said surface for charging the latter.
 5. In a liquid jet recorder as defined in claim 1 wherein said capillary is mechanically vibrated at a high frequency by vibrator means coupled to said nozzle. 